Molecular hybridization of cellular RNA populations with unique DNA sequences and with DNA complementary to the RNA is being employed as a measure of gene expression in several model systems from the nervous system. The studies are directed toward an understanding of the control processes which regulate normal differentiation and development in the nervous system. (a) Further investigation of the effects of bromodeoxyuridine treatment on the nuclear RNA diversity of the neuroblastoma clone S20-DB6 showed that treated cells retained 2/3 of the sequences present in untreated cells and the remaining sequences were expressed as a consequence of the treatment. (b) The sequence diversity of poly(A)-containing messenger RNA in the rat glial cell line C6B was found to be similar to that in neuroblastoma cell lines. (C) A comparison of the poly(A)-mRNAs of differentiated and undifferentiated neuroblastoma cells showed that 15% of the sequences were unique to the differentiated state.